The present invention relates to a duo-servo drum brake for vehicles and more particularly to a technique of stabilizing variation in braking force of a duo-servo drum brake which is mechanically operated.
Various types of drum brakes have heretofore been employed for controlling the traveling of vehicles. These drum brakes are classified into the following: a leading trailing type; a two leading type; a duo-servo type and so forth, depending on the arrangement of brake shoes which are pressed against the inner peripheral surface of a drum.
Of these types of drum brakes, the duo-servo drum brake is generally equipped with two brake shoes disposed opposite to each other; namely, a primary shoe and a secondary shoe. The primary shoe has an input portion on its rearward side in the direction in which the drum is rotated forward and is connected to the rearward side of the secondary shoe via an adjuster. Further, the forward side of the secondary shoe is brought into contact with an anchor pin mounted on a backing plate, so that counter braking force acting on the primary and secondary shoes is received by the anchor pin.
When the primary and secondary shoes are thus expanded and pressed against the inner peripheral surface of the drum, the counter braking force acting on the primary shoe is input to the rearward side of the secondary shoe to press the secondary shoe against the drum, whereby the primary and secondary shoes simultaneously function as a leading shoe and make available braking force having an extremely high gain.
In comparison with those of the leading trailing type and the two leading type, the aforesaid duo-servo drum brake has various advantages in that extremely high braking force is obtainable and that it can be made compact and incorporated as a parking brake. On the other hand, there still exist problems arising from not only its sensitiveness to variation in the friction coefficient of lining of the brake shoe but also difficulty in stabilizing braking force.